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No, that's true, but a CVT, just like a geared transmission, just like any transmission that I know of, has an input shaft, an output shaft, and a means of mechanically coupling the two, combined with a means of varying the ratio of relative speeds that the two shafts turn at. Practical transmissions that I know of have clear limits to the ratios of these shaft speeds - if it's a geared transmission, you have the ratio in the tranny in first gear, and the ratio in the tranny in top gear. If it's a CVT, you have the lowest accessible ratio of the pulley diameters acting as your lower limit, and the highest accessible ratio of pulley diameters acting as the upper limit.
If we ignore the complications of the internals of any particular transmission, and just consider an abstract transmission of "perfect" design subject to physical limits... and then you start off the line at idle and floor the car, the transmission should run in the lowest shaft ratio available until you reach the engine RPM yielding maximum acceleration - that's the initial linear segment you see in the green curve. Note that this actually coincides with the dashed purple line - the geared transmission and the CVT should do the same thing over this portion (subject to malpractice by the engineers of course).
Where it gets interesting is when you reach the RPM of max acceleration (which I *think* is also the RPM of max torque). The perfect transmission will simply level off and stay there while the car accelerates, which is what the CVT can do, and which is what the CVT does do! The geared transmission is incapable of doing that. It just approximates this ideal function with a set of steps - each step is another fixed gear with its own straight line representing another fixed ratio of engine speed to vehicle speed, so you get a zig-zag approximation to the curve. The number of ascending segments is equal to the number of gears in the geared transmission, and the more gears (segments) you add, the closer you can come to that ideal curve. But the CVT is already there!
And you can make this same argument, whether you want max power output, max torque, max fuel efficiency.
Last edited by OutdoorLover; 06-18-2019 at 07:41 PM..
So that's fair, I am not saying that everyone has to prefer them, I am only saying though that all these claims that it's weak, slipping, has a rubber band in it, is slow, etc are just off - those are wrong interpretations of the sounds that people hear. If someone says, well, I just don't like that sound, fair enough.
But, as I posted earlier today, there is a reason Toyota started using an actual launch gear with their CVTs. Surely they must have gotten a lot of complaints about laggy take off.
Where it gets interesting is when you reach the RPM of max acceleration (which I *think* is also the RPM of max torque).
It's actually the RPM of max horsepower. Torque (also called "moment of force") is the twisting force on the crankshaft. However that's not useful information on its own.
Consider this:
Engine A has 200ft-lbs of torque and a redline of 3000rpm. Assume it "breathes" adequately out to redline.
Engine B has the same 200ft-lbs of torque and a redline of 6000rpm.
Which engine will get you to 60 faster?
The answer is, engine B. Consider why for a moment.
Engine A and B, if both connected to the same conventional automatic, will both accelerate equally in first gear. However, engine A will have to shift to the next gear much earlier, meaning you can't continue accelerating at that rate for as long.
If engine A and B are both connected to the same CVT, both will accelerate equally to redline at the CVTs lowest ratio, and then acceleration will begin to drop off as the CVT changes ratios. Engine A will need a much taller ratio at any given speed, and thus will not be able to accelerate as quickly.
This is why horsepower is the important metric for acceleration, not torque. Torque is just one component of horsepower. If two engines are geared "properly", the one with higher horsepower will always out-accelerate the one with less, even if it has much less torque. An engine which builds its horsepower from a wider RPM range just needs different gearing to best extract that power.
Quote:
Originally Posted by southernnaturelover
But, as I posted earlier today, there is a reason Toyota started using an actual launch gear with their CVTs. Surely they must have gotten a lot of complaints about laggy take off.
If I were to speculate, it probably has more to do with reliability, packaging, or the maximum spread of ratios achievable. This is Toyota we're talking about.
But, as I posted earlier today, there is a reason Toyota started using an actual launch gear with their CVTs. Surely they must have gotten a lot of complaints about laggy take off.
I wouldn't be so sure. Automakers are in the business of selling cars, more than they're in the business of designing the best cars. Often, those two goals would be aligned. But sometimes they're not. So the fact that Toyota is doing that, doesn't prove it's better. I think it proves that Toyota believes that customers will prefer it. More than one automaker has been working on making CVTs that act more like conventional automatics. But I've seen zero engineers who claim the result is better, in fact everyone I've seen describe that does so with some mixture of amusement, irritation and exasperation. The simple answer is, it's worse, in engineering/optimality terms, but it may be better, in sales terms.
Engines do not develop constant power at all speeds; they have specific speeds where torque (pulling power), horsepower (speed power) or fuel efficiency are at their highest levels. Because there are no gears to tie a given road speed directly to a given engine speed, the CVT can vary the engine speed as needed to access maximum power as well as maximum fuel efficiency. This allows the CVT to provide quicker acceleration than a conventional automatic or manual transmission while delivering superior fuel economy.
Disadvantages of the CVT
The CVT's biggest problem has been user acceptance. Because the CVT allows the engine to rev at any speed, the noises coming from under the hood sound odd to ears accustomed to conventional manual and automatic transmissions. The gradual changes in engine note sound like a sliding transmission or a slipping clutch -- signs of trouble with a conventional transmission, but perfectly normal for a CVT. Flooring an automatic car brings a lurch and a sudden burst of power, whereas CVTs provide a smooth, rapid increase to maximum power. To some drivers this makes the car feel slower; in fact a CVT will generally out-accelerate an automatic.
Advantages of the CVTEngines do not develop constant power at all speeds; they have specific speeds where torque (pulling power), horsepower (speed power) or fuel efficiency are at their highest levels.
I know you didn't write this originally, but this is a great example of a misunderstanding of what torque and horsepower are.
An engine's greatest "pulling power" will always be in the lowest gear of the attached transmission, with the engine spinning at the highest RPM (with possible exceptions). For the greatest pulling power at a given speed, it will be the engine spinning at the highest possible RPM gearing allows at said speed.
Drivetrains are systems, and the single variable "torque" is meaningless without discussing RPM and gearing if one wants to examine system output.
I know you didn't write this originally, but this is a great example of a misunderstanding of what torque and horsepower are.
An engine's greatest "pulling power" will always be in the lowest gear of the attached transmission, with the engine spinning at the highest RPM (with possible exceptions). For the greatest pulling power at a given speed, it will be the engine spinning at the highest possible RPM gearing allows at said speed.
Drivetrains are systems, and the single variable "torque" is meaningless without discussing RPM and gearing if one wants to examine system output.
That doesn't sound quite right. Horsepower is directly proportional to both torque and RPMs: HP= k*T*RPMs. But at high RPMs, torque falls off and it can even fall off faster than the RPM increase compensates for. So even peak horsepower, normally doesn't come at peak RPMs, forget about peak torque.
That doesn't sound quite right. Horsepower is directly proportional to both torque and RPMs: HP= k*T*RPMs. But at high RPMs, torque falls off and it can even fall off faster than the RPM increase compensates for. So even peak horsepower, normally doesn't come at peak RPMs, forget about peak torque.
Peak acceleration for this Corolla (just as an example) would be at 7100rpm, not 4000rpm, despite peak torque being at 4000 and only falling after that. For max acceleration or max ability to pull something, you would want to hold the engine at 7100rpm.
(Grabbed a random dyno chart from Google)
Area under the curve is very important if you don't have infinite gearing choice, of course.
No, that's true, but a CVT, just like a geared transmission, just like any transmission that I know of, has an input shaft, an output shaft, and a means of mechanically coupling the two, combined with a means of varying the ratio of relative speeds that the two shafts turn at. Practical transmissions that I know of have clear limits to the ratios of these shaft speeds - if it's a geared transmission, you have the ratio in the tranny in first gear, and the ratio in the tranny in top gear. If it's a CVT, you have the lowest accessible ratio of the pulley diameters acting as your lower limit, and the highest accessible ratio of pulley diameters acting as the upper limit.
If we ignore the complications of the internals of any particular transmission, and just consider an abstract transmission of "perfect" design subject to physical limits... and then you start off the line at idle and floor the car, the transmission should run in the lowest shaft ratio available until you reach the engine RPM yielding maximum acceleration - that's the initial linear segment you see in the green curve. Note that this actually coincides with the dashed purple line - the geared transmission and the CVT should do the same thing over this portion (subject to malpractice by the engineers of course).
Where it gets interesting is when you reach the RPM of max acceleration (which I *think* is also the RPM of max torque). The perfect transmission will simply level off and stay there while the car accelerates, which is what the CVT can do, and which is what the CVT does do! The geared transmission is incapable of doing that. It just approximates this ideal function with a set of steps - each step is another fixed gear with its own straight line representing another fixed ratio of engine speed to vehicle speed, so you get a zig-zag approximation to the curve. The number of ascending segments is equal to the number of gears in the geared transmission, and the more gears (segments) you add, the closer you can come to that ideal curve. But the CVT is already there!
And you can make this same argument, whether you want max power output, max torque, max fuel efficiency.
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There are NO gears.
